Plasma produced during laser irradiation emits light of certain wavelength according to a material, and therefore, constituents of the material can be qualitatively or quantitatively analyzed by collecting the light. Laser-induced breakdown spectroscopy (hereinafter, LIBS) as one of the methods analyzing constituents of a material using the collected light is a spectrum analysis technique using plasma, which is produced by causing breakdown, a kind of discharge phenomenon using high power laser, as an excitation source. A sample is evaporated in the plasma induced by laser, and atoms and ions can exist in an excited state. The excited atoms and ions emit energy after certain life, and then go back to the ground state. At this time, they emit their own wavelengths according to the kind and the excited state of the atoms. Therefore, constituents of a material can be qualitatively or quantitatively analyzed by analyzing a spectrum of the emitted wavelength.
FIG. 1 is an exemplary diagram representing the operation principle of LIBS according to the conventionally technique.
Referring FIG. 1, first of all, as shown in step (1), after the infinitesimal amount (several μg) of a material is subjected to ablation (removal of material by melting and vaporization caused by laser) by irradiating pulse laser, the ablated material is ionized within very short time (commonly, within several ns) by absorbing the laser energy so as to form plasma having high temperature of about 15000 K or more as shown in step (2). After completing laser pulse, each atom in the plasma expresses its own spectrum while the high temperature plasma is being cooled. The generated spectrum is collected using an analysis apparatus shown in step (3), and analyzed to obtain unique spectrum data of each atom. Finally, the composition and amount of the ingredients in the material can be measured by analyzing the data.
LIBS technique is distinguished from other analysis techniques in that {circle around (1)} time consumed to total less than 1 sec, {circle around (2)} separate sampling and pre-treatment processes are not needed for the analysis, {circle around (3)} atomic constitution of the material can be analyzed with nm unit precision while ablating the material in depth because only infinitesimal amount (several μg) of material is required for one analysis, {circle around (4)} separate environment is not needed for the analysis, and the analysis can be conducted in air, {circle around (5)} every atoms except for inert gases can be analyzed with ppm precision, and {circle around (6)} equipments can be made up at relatively low cost.
FIG. 2 is a diagram showing the result of comparing LIBS and other measuring technique.
Referring to FIG. 2, methods commonly used to measure material distribution such as SIMS (Secondary Ion Mass Spectrometry), AES (Atomic Emission Spectroscopy), EDS (Energy Dispersive X-ray Spectroscopy), GD-MS (Glow Discharge Mass Spectrometry) and the like measure the distribution only in a laboratory level but can't be actually applied to a production line because they need high vacuum. Besides, ICP-MS (Inductively Coupled Plasma-Mass Spectrometry), which is broadly being used, can't be applied to the production line because it has difficulty that a sample to be analyzed should be dissolved in a solvent before analysis. Now, XRF (X-ray Fluorescence), which is mostly used to the analysis of a solar cell material at a laboratory or site due to its convenience in use, has an advantage that the analysis can be conducted in air at relatively low cost, but has technical limits to analyze the material distribution of a CIGS thin film in that {circle around (1)} it is impossible to measure the amount of Na in a CIGS thin film, which has decisive influence on the device efficiency, is impossible because analysis of light weight atoms such as Na, O, N, C, B, Be, Li and the like is almost impossible, {circle around (2)} it is impossible to measure the atomic distribution in depth in a CIGS thin film having the thickness of 2 μm because the precision of XRF in depth is only about 1 μm at most, and {circle around (3)} it is difficult to distinguish whether the measured fluorescence signal is from an actual thin film or from a substrate.
In general, a semiconductor solar cell is defined as a device directly converting sunlight to electricity using photovoltaic effect, wherein electrons are produced when light is irradiated to a p-n junction semiconductor diode. As the most basic constitutional elements, it is divided to three parts such as a front electrode, rear electrode and absorber layer located therebetween. Among theses, the most important material is the absorber layer, which decides most of the photoelectric conversion efficiency, and a solar cell is divided to various kinds according to the material. Particularly, when the material of the absorber layer is composed of I-III-VI2 compound such as Cu(In,Ga)Se2, it is called a CIGS thin film solar cell, and the CIGS thin film solar cell, a high-efficient and cheap solar cell, is receiving attention as the most firm second-generation solar cell to replace a crystalline silicon solar cell in a solar cell field where recently fierce competition is taking place all over the world, and it shows the highest efficiency of 20.6%, which is the most close efficiency to a single crystal silicon device.
FIG. 3 is an exemplary diagram schematically representing a structure of a CIGS thin film solar battery as one application area of the present invention.
FIG. 4 is a flow chart schematically representing a production process of a CIGS thin film module.
First of all, a CIGS thin film solar cell is prepared by sequentially depositing Mo layer, CIGS layer, CdS layer and TCO layer on a substrate, and it is more specifically prepared in detail as follows. The CIGS thin film module is prepared by, first of all, depositing Mo as a rear electrode layer on a substrate; forming a pattern by a scribing process (P1 scribing); sequentially depositing CIGS layer as an absorber layer and CdS buffer layer on the pattern-formed Mo layer; forming a pattern by a scribing process (P2 scribing); depositing again TCO (transparent conductive oxide) layer on the CdS layer followed by depositing a front electrode grid of Ni/Al; and then finally proceeding a scribing process to form a pattern (P3 scribing). The said scribing process is a patterning process to serially connect the patterns at regular intervals in order to prevent the efficiency reduction caused by increase of the sheet resistance with increased area of the solar cell, and the process is conducted via three times of P1, P2 and P3. Conventionally, the P1 scribing was patterned by laser, and the P2 and P3 scribing were patterned by a mechanical method, but recently, a method using laser to pattern all of the P1, P2 and P3 scribing is being developed.
In case of this CIGS thin film solar cell, it is being reported that not only the thickness of the thin film (1˜2.2 μm) or the device structure but also the composition of the constituent material of the CIGS thin film as a multi-component compound and atomic distribution in the thin film have critical influence on the light absorption rate and photoelectric conversion efficiency. Further, it is being reported that Na, which is diffused from a soda-lime glass largely used as a substrate to a CIGS absorber layer during a process, increases the photoelectric conversion efficiency by increasing the electric charge concentration of the thin film (Nakada et al., Jpn. J. Appl. Phys., 36, 732 (1997)), or by increasing the grain size of the CIGS single crystal so as to reduce the structural characteristic change according to the composition change (Rockett et al., Thin Solid Films 361-362 (2000); Probst et al., Proc. of the First World Conf. on Photovoltaic Energy, Conversion (IEEE, New York, 1994), p. 144). These reports suggest that the chemical property of the absorber layer should be controlled through the distribution analysis of a material in the thin film for quality control at the CIGS thin film solar cell production line.
On the other hand, the continuous production process of the CIGS thin film solar cell is largely divided to a Roll-to-Plate (hereinafter, R2P) process using a hard material substrate such as a soda-lime glass, and a Roll-to-Roll (hereinafter, R2R) process using a soft material substrate such as metal thin plate (e.x., stainless steel, Ti, Mo, Cu and the like) or polymer (e.x., polyimide). Now, the physicochemical property should depend on the previously decided value in the research and development step because a system, which can analyze the physicochemical properties of the CIGS thin film having strong influence on the product performance in real-time, is not equipped yet in these continuous production lines. Further, it is impossible to check separately even if the property is out of the physicochemical standard desired in the actual production process, and therefore, the error should be found out through the decrease of the performance and quality in the evaluation step of the finally completed product, and great product loss is generated. Because many efforts and time are consumed to find out physicochemical variables causing falling off in product performance and quality in the said continuous production process, cost increase and falling off in competitiveness are caused consequently. Therefore, development of a process control system, which can analyze the physicochemical properties of a produced CIGS thin film in real-time in the continuous production process line without a pre-treatment process, is desperately needed.